It is known that many gases absorb radiation particularly in the infrared wavelength range, wherein this absorption occurs at a wavelength which is characteristic for the gas in question, for example for CO2 at 4.24 μm. With the aid of infrared gas sensors it is thus possible to establish the presence of a gas component or the concentration of a gas component.
Known infrared gas sensors have, for example a broadband radiation source, an absorption path or a measuring chamber, a wavelength-selective element, for example an interference filter, a Fabry-Perot filter or a grating and a radiation detector, for example a pyroelectric, a PbSe or thermophilic detector. The radiation intensity measured by the radiation detector is a measure of the concentration of the gas absorbing at the set wavelength. Instead of the broadband radiation source and the wavelength-selecting elements, the use of a selective radiation source, such as for example an LED or a laser in combination with non-wavelength-selective radiation receivers, is also known.
Photoacoustic gas sensors are also known. The operating principle of a photoacoustic gas sensor is based on the detection of the pressure change, which is produced by heating the gas molecules in the measuring chamber, by means of an acoustic detector, such as for example a microphone.
Heating is caused by the absorption of the radiation energy of the radiation source by the measuring gas molecules to be detected. One design of a photoacoustic gas sensor corresponding to the state of the art can be seen, for example from German patent 19 525 703.
The long-term stability of such infrared and photoacoustic gas sensors depends essentially on the aging of the radiation sources and on the soiling of the overall optical system. Soiling is currently prevented by suitable gas-permeable filters for keeping out contaminating particles.
To monitor the aging of the radiation source, the gas sensor or the gas sensors is periodically controlled with respect to zero point and sensitivity drift. Hence, the gas sensors are rinsed with a gas or gas mixture without the gas component to be measured in order to determine the position of the zero point. After zero-point control, they are rinsed using a gas mixture of known concentration of the gas component to be measured, wherein the measured value obtained is compared with a preset value. This calibration usually has to be carried out by trained personnel and is thus very cost-intensive.
A further possibility for monitoring aging of the radiation source of an infrared gas sensor consists in using a second radiation detector with an optical filter for the detection of the radiation intensity at the wavelength, at which a gas component of the measuring gas does not have considerable absorption (see U.S. Pat. No. 5,341,214). By forming a quotient of the two signals obtained, that is measuring signal to reference signal, aging of the radiation source is compensated. Since the detectors represent a main cost factor of the gas sensors and since double signal processing is required, such gas sensors are relatively expensive. In addition, different drift for the detectors may occur during temperature changes.
Furthermore, there are infrared gas sensors with two radiation sources, in which the light path of the one radiation source does not go through the measuring gas chamber, with which soiling of the optical system and aging of the other radiation source may be determined. However, according to principle this is only possible using a reference gas without the gas component to be measured.
Also, an arrangement is known where two light sources are arranged within the gas chamber at a different distance from the radiation detector. The two radiation sources are modulated at different frequencies and the detector signal is demodulated. The output signals of the different frequencies are divided by one another, so that the distance between both sources produces the effective light path. However, due to the fact that the radiation sources do not age uniformly (see FIG. 10), there may be an intensification of the drift of the sensor due to aging of the radiation sources.